1. Field of the Invention
The present invention relates to systems for connecting remote computer systems to computer networks through telephone lines, and more particularly to a modem which operates at a voice band frequency below 4000 Hz as well as a higher broad band frequency above 4000 Hz.
2. Related Art
With the advent of computer networking and personal computers, public switched telephone networks (PSTNs) are increasingly used to connect computer systems to other computer systems, and to connect computer systems to computer networks, such as the Internet. As computer systems begin to support data-intensive applications, which transfer sounds, images and video between computer systems, the connections between computer systems are subject to increasing demands for larger amounts of bandwidth. Although the subscriber loops, which connect telephones to central office switching systems, are capable of accommodating transfers of data between computer systems, the available bandwidth of data transfers through PSTNs is severely constrained by inherent limitations of the switching equipment located in central office switching systems. Most of this switching equipment digitizes analog signals on subscriber loops by sampling the analog signals at a frequency of 8 kHz. Because of the Nyquist limit, this sampling rate imposes an artificial maximum frequency of 4 kHz on signals that can be passed from a subscriber loop through a PSTN.
Fortunately, the majority of capital investment in the telephone system is in the subscriber loops, which are pairs of wires running to homes and business from telephone central office switching systems. With new asymmetric digital subscriber loop (ADSL) modems, a typical subscriber loop can send 8,000,000 bits per second. The main difference between ADSL modems and older V.34 type modems is that in ADSL a distant modem is located in a telephone central office, and it receives a signal across a subscriber loop before the bandwidth is artificially reduced by equipment in the central office switching system. (In the following disclosure, the term "ADSL" is used as a generic description of all modems operating over direct telephone wire at frequencies above 4,000 Hz. The actual method used to modulate the line at the high band rate may or may not actually be asymmetric.)
ADSL technology was largely started by Dr. Joseph W. Lechleider at Bell Communications Research (Bellcore) during the mid-1980's, and has culminated in American National Standards Institute (ANSI) standard T1.413. The same standard has been adopted by the European Telecommunications Standard Institute (ETSI). The T1.413 standard is based upon discrete multi-tone (DMT) modulation. There are other related techniques, which are not standardized, such as Carrierless Amplitude modulation/Phase modulation (CAP), and another older method known as Quadrature Amplitude Modulation (QAM). The three above-mentioned methods can be grouped together has efficient line codes. This means that they are capable of approaching the Shannon-Hartley limit for the maximum amount of information which can be sent through a given channel. In addition to these three methods, there are many older modulation techniques such as AMI (Alternate Mark Inversion) and 2B1Q, which operate at levels below the maximum possible rate. All of these codes can be used to send high speed data over ordinary telephone lines.
While the theoretical capabilities for ADSL technology are quite promising, there exist large numbers of barriers to making the required changes to implement ADSL technology on a wide scale. First, the technology required to implement ADSL modems is considerably more expensive than for existing V.34 modems. For example, an ADSL modem at this time costs about $2,000, compared to about $100 for a V.34 modem. ADSL modems can be expected to decline in price to somewhere in the range of $500 to $1,000 within the next year because of volume shipments. However, there will nevertheless exist an approximately 5 to 1 ratio in cost of ADSL over V.34 modems for the foreseeable future. Second, there is currently no infrastructure to support ADSL modems in telephone central offices. A large number of ADSL modems, all requiring 8 Mbits per second of data, will quickly overwhelm any conceivable data switch of today's technology. Because of this bandwidth bottleneck, it is unlikely that the benefits of ADSL technology will actually be realized on a broad scale any time soon. Unfortunately, the tremendous additional cost for ADSL modems must be paid even though much of the additional data rate will not be immediately useable.
Another problem with ADSL is that there is no known content which requires such bandwidth. Video on demand (VOD) could theoretically utilize the bandwidth available through ADSL technology however, VOD requires a complex and expensive system of video servers to distribute the material. This infrastructure presently does not exist. Internet access is a rapidly evolving content area which could theoretically utilize the additional bandwidth of ADSL technology, but at present, Internet applications are designed for transmission facilities comprising 33 Kbit links and T1 trunks.
What is needed is a modem which is capable of providing the increased bandwidth from ADSL technology at a fraction of the cost.